The present invention relates to a method of sintering sulfidic metals to metallic oxides. The metallic oxides are suitable as feed material to a blast furnace for the reduction of the metallic oxides and recovery of the elemental metal. More specifically, the present invention relates to a method of enhancing the sintering operation comprising treating the sinter mix with an aqueous transition metal sulfate.
In sintering, a shallow bed of fine particulates is agglomerated by heat exchange and particle fusion. Heat is generated by combustion of a solid fuel admixed with the bed of mineral bearing fines being agglomerated. The combustion is initiated by igniting the fuel exposed at the surface of the bed, after which a narrow, high temperature zone is caused to move through the bed by an induced draft usually applied at the bottom of the bed. Within the narrow combustion zone, the surfaces of adjacent particles reach fusion temperature and gangue constituents form a semi-liquid slag. The bonding is effected by a combination of fusion, grain growth and slag liquidation. The generation of volatiles from the fuel and fluxstone creates a frothy condition and the incoming air quenches and solidifies the rear edge of the advancing fusion zone. The product consists of a cellular mass of ore bonded in a slag matrix.
In the iron industry, the essential materials for sintering consist of a mixture of iron-bearing fines and a solid, particulated fuel. The iron-bearing constituents are principally iron fines, recycled sinter fines, and flue dust but may also include mill scale, open hearth precipitator dust, dust from basic oxygen steel production (BOP) and similar iron-bearing materials. Coke breeze is the most common solid fuel, but other carbonaceous materials can be used. It has also become common practice to incorporate limestone fines into the sinter mix. This composition of fine materials is well mixed and placed on the sinter strand in a shallow bed, seldom less than 6 inches or more than 20 inches in depth. Upon ignition, within a furnace which straddles the bed, the surface of the bed is heated to about 1269° to 1370° C., combustion of the fuel is initiated and the fine particles at the surface are fused together. As air is drawn through the bed, the high temperature zone of combustion and fusion moves downward through the bed and produces a bonded cellular structure.
During the process, the induced air is preheated by the hot sinter overlaying the combustion and fusion zone, and the sensible heat contained in the combustion products and in the excess air is transferred to the bed below the fusion zone.
The detailed design and physical placement of sintering equipment and the flow pattern of materials may differ considerably among various plants. The choice of equipment is generally based upon desired capacity, space availability, capital costs, the materials to be handled, and prevailing technology. Each plant can however, be subdivided into three distinct phases of operation. These are 1) raw material processing, 2) sinter production, and 3) product processing.
In the raw material processing operation, iron ore undergoes screening and benefication, typically at the mine, and the ore feed of −10 millimeters is accepted for sinter making. A separate raw materials system handles the balance of the materials, such as flue dust, fluxes, coke breeze, plant wastes, etc. The additives including coke breeze are crushed to a size range below −3 to −15 for preparation of a sinter mix and conveyed directly to storage bins. From the raw materials in storage bins, the desired materials are fed at controlled and specified rates onto a common collector belt. In order to provide homogeneity, bedding and blending of the raw materials is used in order to provide a definite proportion of materials for the sinter bed. Moisture for proper conditioning of the mix is added during the mixing and conditioning. The mixed and micro-pelletized feed is transferred to the sinter strand.
The production of the sinter per se occurs entirely on the sinter strand. Prior to feeding the raw mix, a grate layer of cold intermediate size sinter, usually 15-40 millimeters is fed onto the machine usually to a depth of 25 to 50 millimeters. This is done to reduce the temperature to which the grate bars are exposed; lower temperatures extend grate bar life. This layer also suffices to reduce the amount of fine material reaching and passing through the grate bars. The raw mix is fed directly onto the grate layer to a predetermined bed depth usually 300 to 500 millimeters. This is ignited by a furnace fired with a liquid or gaseous fuel, and the process of sintering is initiated. The speed of the machine (sinter strand) is regulated such that the high temperature zone of fusion reaches the grate layer as the material reaches the discharge end of the machine.
After ignition, suction fans pull air through the sinter bed into wind boxes located under the sinter strand and then into one or more collection mains essentially causing the forced air ignition and fusion of the sinter mix. Dust cleaning of the exhaust gases is done in cyclone separators or electrostatic precipitators.
Product processing begins at the discharge of the strand where the porous coherent lumps pass through a breaker. The large cake is reduced to a maximum size of 200 to 300 millimeters to facilitate cooling. The fines generated from this crushing operation are removed by a screen and recycled to the raw mix feed. The oversize is conveyed to sinter cooler. These are usually devices for passing air through the hot sinter bed. The purpose is to reduce the temperature of the sinter such that it maybe subsequently handled without damage to the conveying equipment. From the cooler, the sinter is cold screened, usually into three sizes. The smallest size, usually minus 5 millimeters, is recycled as cold return fines. An intermediate size, usually 10 millimeters by 40 millimeters, is either recycled as grate layer or sent to product storage depending on the needs of grate layer material. The coarsest size is sent directly to product storage.